1. Technical Field
The present invention relates to a semiconductor integrated circuit, and more particularly, to an oscillation circuit of a semiconductor apparatus capable of adjusting its duty ratio.
2. Related Art
In general, a semiconductor apparatus requires an oscillation circuit oscillating at a constant operating frequency. In particular, some circuits that require an oscillation circuit oscillating at a constant operating frequency are a power circuit driving a pump for generating a boosted voltage (VPP) or a back bias voltage (VBB) and a PLL circuit determining an oscillation frequency in proportion to an input voltage.
FIG. 1 is a circuit diagram illustrating a conventional oscillation circuit.
As illustrated in FIG. 1, the oscillation circuit 10 is a ring oscillator configured as follows: an output of a first inverter IV1 is coupled to an input of a second inverter IV2; an output of the second inverter IV2 is coupled to an input of a third inverter IV3; an output of the third inverter IV3 is coupled to an input of a fourth inverter IV4; an output of the fourth inverter IV4 is coupled to an input of a fifth inverter IV5; and the fifth inverter IV5 outputs an oscillation signal OSC and the output of the fifth inverter IV5 is coupled to the input of the first inverter IV1. Generally an odd number of inverters are coupled together.
The operation of the conventional oscillation circuit 10 of FIG. 1 will be described below in detail. When the present output of the fifth inverter IV5 is at a high voltage, the high voltage is applied to the input of the first inverter IV1 and is inverted. Thus, the output of the fifth inverter IV5 is at a low voltage. Again, the low voltage is applied to the input of the first inverter IV1 and is inverted. Thus, the output of the fifth inverter IV5 is at a high voltage. In this manner, the conventional oscillation circuit outputs the oscillation signal OSC. By repeating such operations, the oscillation signal OSC having a constant period is outputted. The oscillation period of the oscillation signal OSC is controlled through RC components.
In the conventional oscillation circuit 10, the oscillation period is varied if a power supply voltage (VDD) is varied. Specifically, as the power supply voltage (VDD) decreases, the oscillation period becomes longer. As the power supply voltage (VDD) increases, the oscillation period becomes shorter.
That is, the conventional oscillation circuit 10 has difficulty in obtaining an accurate oscillation period because it is sensitive to variations in process, voltage, and temperature (PVT).